Abstract
Bottom trawling for crustaceans in Portuguese coastal waters is an important fishery in terms of revenue, despite its negative impacts on deep-sea ecosystems. This fishery catches large amounts of unwanted species that were discarded for various reasons before the introduction of the Landing Obligation, which banned the discarding of regulated species. However, where it can be demonstrated that a species has an acceptably high likelihood of survival, exemptions to this ban may be granted. In this study, time-to-mortality was used to estimate immediate mortality rates and identify important biological characteristics determining the susceptibility of 14 by-catch species, most with commercial interest (Conger conger, Galeus melastomus, Helicolenus dactylopterus, Lepidorhombus boscii, Lophius budegassa, Lophius piscatorius, Merluccius merluccius, Micromesistius poutassou, Mullus surmuletus, Phycis blennoides, Scyliorhinus canicula, Trigla lyra, Trachurus trachurus and Trachurus picturatus). Species with potential to survive after the discarding process were identified and a short-term survival assessment of conger eel (Conger conger) was performed. The results suggest that species with scales, gas bladder and high metabolic rates have higher post-discard mortality. Size was a critical factor determining survival in some species, with smaller individuals dying faster. The short-term survival rate of conger eel was determined to be 84% (95% CI: 75.5 to 93.3%). The methodology and results from this study can help identify species that may survive the discarding process and factors influencing their survival.
Highlights
Despite its impact on marine ecosystems through the modification of sea-bottom morphology and resuspension of sediments, bottom trawling is one of the most common fishing practices around the world (Pusceddu et al 2014, Clark et al 2015)
If scientific evidence demonstrates that a species has an acceptably high survival when released from a particular fishery, discarding of that species may be permitted (EU 2013)
T. picturatus, Lophius piscatorius and L. budegassa were grouped by genera, i.e. mackerel species and monkfish species, respectively
Summary
Despite its impact on marine ecosystems through the modification of sea-bottom morphology and resuspension of sediments, bottom trawling is one of the most common fishing practices around the world (Pusceddu et al 2014, Clark et al 2015). Demersal trawl nets collect an extensive range of species and sizes and have high discard rates. “The discard problem” raises several issues, including policy and ethical implications, fisheries management, and ecological, economic and technical concerns (Kelleher 2005). One of the key changes during the recent reform of the Common Fisheries Policy was the intention to “gradually eliminate the wasteful practice of discarding” (EU 2013). A discard ban (the “Landing Obligation”) obliges fishermen to keep and land all regulated species caught: i.e., species with quotas (total allowable catches) or minimum conservation reference sizes in the Mediterranean. If scientific evidence demonstrates that a species has an acceptably high survival when released from a particular fishery, discarding of that species may be permitted (EU 2013)
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